CA2053947A1 - High performance triangle interpolator - Google Patents

Abstract

r9-90-022 ABSTRACT

A graphics processor including an interface for providing triangle primitives representing a graphics image, a triangle interpolator coupled to the interface for interpolating a triangle primitive and serially computing multiple line primitives from the triangle primitive, a line renderer coupled to the triangle interpolator for receiving a line primitive from the triangle interpolator and for providing pixels representing the line primitive while the triangle interpolator is computing another line primitive.

Description

~9-90-022 1 2~3~47 HIGH PERFORMANCE TRIANGLE INTERPOLATOR

DESCRIPTION

elated P _ent Applications Related patent applicatidhs include copending application U.S. Serial No. filed Nové~be~
15, 1990, entitled "High Performance Rasterizâtio~
Engine" (IBM Docket No. AT9-89-107); copendln~
application U.S. Serial No. filed Nov~be~
15, 1990, entitled "High Performance Bitblt and Line ~ra~
Enginel' ~IBM Docket No. AT9-90-055), COPehdi application U.S. Serial No. filed Novem~e~
15, 1990, ehtitled 'IHierarchical ~emory Controller" (~B~
Docket No. AT9-90-054)~ and cop~nding application U.S~
Serial No. _ filed Nov~ber 15, lg90, entltled i'I/O Regist~r Protection Circ~it" (IBM Docket ~o.
AT9-90-001).

Technical Field The present invention relates generally to compUte~
graphics cards and more specifically to a ~lgh performance rasterization processor.

Backqround Art In a typical scalar computer, the computer include~
a main processor coupled to a memory, an input device and an output device. Input device may include a keybo~rd, mouse, tablet or other types of input devices. output device may include a text monitor, plotter or other type~
of output devices. The main processor may àls~ b~
coupled to a graphics ou~put de~ice such as a graphics display through a graphics card. Graphics card rec~é~
instructions reyarding graphics from main processor. Th~
graphics card then executes those instructions and generates Red/Green/Blue (RGB) signals to the graphic~
display thereby renderiny the desired graphics output from the work-station processor.

T9-90-022 2 2~39~7 Disclosure of the Invention The present invention provides a graphics processor including an interface for providing triangle primitives representing a graphics image, a triangle interpolator coupled to the interface for interpolating a triangle primitive and serially computing multiple line primitives from the triangle primitive, a line renderer coupled to the triangle interpolator for receiving a line primitive from the triangle interpolator and for providing pixels representing the line primitive while the triangle interpolator is computing another line primitive.
A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.

Brief Description of the Drawing Fig. 1 is a block diagram of a typical scalar computer;
Fig. 2 is a block diagram of a preferred rasterization processor and display controller;
Fig. 3A is a diagram which shows the relationship between the parameters in the triangle data structure and the rendered triangle primitive;
Fig. 3B is a diagram of a pre~erred embodiment of the -triangle interpolation subsystem;
Fig. 3C is a timing diagram which shows the parallelism achieved by the triangle interpolator;
Fig. 4A is a block diagram of the bitblt and line draw parameter calculator dataflow;
Fig. 4B is a state flow diagram of the bitblt and line draw parameter calculator;
Fig. 5 is a block diagram of the bitblt and line draw sequencer dataflow; ~:
Fig. 6A is a more detailed block diagram of the input interface shown in Fig. 2;
Fig. 6B is a state diagram of the input interface shown in Fig. 6A;
Fig. 7A is a more detailed block diagram of the memory hypervisor shown in Fig. 2;

~ ~rg_gO-022 20~3947 Figs. 7B and 7C show how the memory hypervisor of Fig. 2 would process a series of writes to the frame buffer; and Figs. 8A through 8D are timing diagrams that show how the rasterization processor and display controller of Fig. 2 would process various types of image primitives.

Best Mode for Carryin~ut the Invent _n Fig. l is a black diagram of typical scalar computer 100. In a preferred embodiment of the invention, graphics card 200 includes a front end graphics processor 210, a rasterization processor and display controller 220 (also known as a rasterizer or a rasterization engine), a frame buffer 230, a Z buffer 240, an attribute buffer 250, and a Random Access Memor~ Digital Analoge Converter (RAMDAC) 260. The frame buffer and the attribute buffer are usually Video Random Access Memory (VRAM) and the Z
buffer is usually Dynamic Random Access Memory (DRAM).
The front end graphics processor 210 recéives instructions from the main processor regarding a graphics construct or image in world coordinates and described generally by what are called image primitives which include triangles, bitblts, lines, etc. The front end graphics processor can also handle pixel instructions from the workstation processor. The front end graphics processor then performs various transormation, clipping and lighting instructions thereby describing the graphics construct or image in near screen coordinates. This information is then passed on to rasterizer 220.
Rasterizer 220 performs an inner loop of the processing of a graphics image or construct. The rasterizer typically generates pixels along a line primitive, copies from one area on a display to another, performing a logical or arithmetic function between two areas on a display (such as an exclusive-OR bitblt), filling a triangle shaped area with shaded pixels after performing a depth comparison test (Z buffered Gouraud shaded triangles), and other typical graphics functions.
Overall, the rasterizer updates the frame buffer, Z
buffer and attribute buffer in screen coordinates based ,:
~,, ; " .

T9-90-022 4 20539~

on the information from the front end graphics processor and the information previously stored in the buffers.
The frame buffer 230, which may be double buffered, includes a description of the red, green and blue colors for each pixel of the graphics display. The Z buffer contains a description of the depth or Z value of the pixels stored in the frame buffer. This information is useful for the rasterizer to determine whether or not a proposed pixel to be displayed based on a logical test.
If the logical test is false, then the frame and Z
buffers are not updated and the current pixel is displayed rather than the proposed pixel. If the logical test is true, then the frame and Z buffers are updated and the proposed pixel is displayed. In the preferred embodiment, the frame buffer has 24 planes (8 planes each for red, green and blue) for 1280 by 1024 pixels and the Z buffer has 24 bits per pixel for the same number of pixels. The attribute buffer 250, which may also be double buffered, includes control information about the pixels in the frame buffer such as which windows, overlays and microcode plane masks they belong to, whether they are write protecte~, etc.
RAMDAC 260 then uses the description of the pixels in the frame buffer and the control planes from the attribute buffer to generate an analog RGB signal stream.
Graphics display 150 then displays tha graphical image using the RGB signal stream.
Fig. 2 is a block diagram of rasterization processor 220. An input interface 300 communicates with the front end graphics processor in a predetermined protocol.
Based on this communication, the input interface decodes addresses, directs data to the appropriate internal registers, and generates interface timings which meet a rasteri~er front end 305 s requirements. For example, the input interface sends information pertaining to triangle primitives to a triangle interpolator 310. In addition, the input interface sends the address information of bitblt and line primiti~es to a bitblt and line draw parameter calculator 320 and the corresponding data to a width and format convertor 330. In the rg-90-022 5 2~53947 preferred embodiment, the input interface determines if the internal registers are free to receive data. If the internal registers are not available, then the input interface write protects the internal reyisters and holds up the data until the appropriate internal registers are free to receive the data. In addition, the input interface read protects internal registers that do not have valid data. The input interface has two-way communication, including status information, with a command interpreter and supervisor 340. The input interface also passes certain CRT control codes to a CRT
controller 350.
Triangle interpolator 310 can be viewed as an address and data splitter. Information pertaining to triangle primitives are sent to the triangle interpolator 310, which the triangle interpolator then separates into a series of line primitives. The triangle interpolator then splits the address information from the data for the line primitives and passes the address information ~o the bitblt and line draw parameter calculator. The triangle interpolator also passes the corresponding data (including color and z information) to a data path 370 in the appropriate width and format. The triangle interpolator can also handle trapezo;.d primitives (quadrilaterals that have upper and lower edges that are parallel to the X axis) in a similar manner.
Bitblt and line draw parameter calculator 320 receives the address information of horizontal line primitives from triangle interpolator 310 and the address information of bitblt and line primitives in any orientation from the graphics processor via input interface 300. The bitblt and line draw parameter calculator then calculates various parameters for the bitblt and line primitives and passes that information to a bitblt and line draw sequencer 360. The bitblt and line draw sequencer then generates incremental pixel addresses for every pixel affected by the bitblt or line primitive. In addition, the sequencer also provides controlli.ng information to data path 370. This information is then passed on to a hierarchical memory `'T9-90-022 6 2a53947 controller 380 (also referred to as a rnemory hypervisor) to update the frame buffer and the Z buffer.
Width and format convertor 330 translates input data from the front end graphics processor or main processor to a format compatible to the targst buffer. The converter handles strings of data for pixels that are contiguous. The data includes color and Z information.
Lines with interpolated color signals do not get passed to the width and format convertor, bu-t are passed directly to the bitblt and line draw parameter calculator 320. In the preferred embodiment, the converter can convert 1 bit pi~els to 2~ bit pixels allowing expansion of monochrome text to any of two colors in the frame buffer. In addition, the width and format convertor 330 can convert 32 bit words to 40 bit words or other width and format conversions necessary for the utilization of the rasterizer. Once the width and format of the incoming data is converted, the data is passed to data path 370.
Data path 370 passes data from triangle interpolator 310 and the width and format convertor 330 to the memory hypervisor 3~0 in cooperation with the bitblt and line draw sequencer 360. This cooperation is coordinated by the command interpreter and supervisor 340. The data path also performs all necessary data manipulation such as pixel rotation and align-ment, as well as source and destination mixing. Command interpreter and supervisor 340 coordinates the action of rasterizer 220 through various control signals. The command interpreter and supervisor also communicates the status of the rasterizer to the graphics processor via the input interface 300.
Cathode Ray Tube (CRT) controller 350 handles various control information for the CRT or display device. This includes horizontal and vertical sync refresh, frame blanking, composite sync, and other control signals that coordinate the actions of the graphics display device through memory hypervisor 380.
The CRT controller also generate~ the addresses and load signals for the operations of the frame buffer.

`T9-90-022 7 2~53947 Memory hypervîsor 380 handles upda-ting the frame buffer, Z buffer, and attribute buffer via frame buffer controller 390, Z buffer controller 392, and attribute buffer controller 394, respectively. The attribute buffer control planes are updated by the memory hypervisor as instructed by the graphics processor. The frame buffer pixels are updated according to a Z
algorithm. That is, a logical test is performed. If the logical test is true, then the frame buffer and the Z
buffer are updated by the new pixel values by the memory hypervisor 380. This Z algorithm is different from a Painter s algorithm wherein the Z values of primitives, rather than pixels, are compared to determine which primitive is in front of the other for display purposes.
The advantage of the separation of bitblt and line draw parameter calculator 320 and the sequencer 360 is so that once the parameters are calculated and passed on to the sequencer, the parameter calculator is then able to handle the next bitblt or line primitive without waiting for the line draw sequencer to complete its processes.
This is particularly useful for handling triangle and trapezoid primitives. That is, the triangle interpolator 3~0 splits the triangle primitives into a series of horizontal line primitives which are passed se~uentially to the bitblt and line draw parameter calculator. The triangle interpolator can then move on to the next triangle line while the bitblt and line draw parameter calculator and the bitblt and line draw sequencer go -through their processes for handling the various horizontal lines.
In the preferred embodiment, the triangle interpolator works at a speed of ten cycles per line primitive split from a triangle or trapezoid primitive, the bitblt and line draw parameter calculator works at a speed of six cycles per line primitive, and the bitblt and line draw sequencer works at a speed of one cycle per pixel. As a result, the length of the line primitives (i.e. the number of pixels) drawn by the bitblt and line ` i draw sequencer directly affects how fre~uently the sequencer can start drawing a line primitive such that . . .
,'', , .~:
: ., .

~3947 `'rg-go-022 8 either the parameter calculator or the sequencer is the one holding up the other. When drawing triangle primitives, the width of the triangle primitive determines the length of the resulting line primitives.
If you use smaller and smaller triangle primitives, then the bitblt and line draw sequencer can keep up with the parameter calculator. This is extremely useful for small triangle calculations such as Gouraud shading. If you tesselate down to one triangle primitive per pixel, then that is roughly equivalent to Phong shading.
The individual subsystems of the rasterizer will now be shown to more fully describe the present invention.
Subsequently, various timing diagrams will be shown to more fully describe how the various subsystems interrelate.

Triangle Interpolator The triangle interpolator can be viewed as an address and data splitter. For e~ample, a Go~raud shaded triangle can be rendered by slicing the triangle primitive into horizontal lines, interpolating along the left edge to calculate the XY values and colors for each line's start, and the right edge to find each line s end point. The triangle interpolator is given the follow-ing 18 parameters from the input interface:
Ymin - y value of the top of the ; triang:Le Ymax - y value of the bottom of the triangle XL - starting x value of the left side of the triangle XR - starting X value of the right side of the triangle ~XL - change in the X value of the left side of the triangle for every y ~XR - change in the x value of the right side of the triangle for every y Ro~GG~Bo~Zo - initial color and depth values, ~Tg-go-022 9 20~3947 defined at [Ymin, Xleft~
~Rs,Gs,~Bs,~Zs,- color and depth deltas along the ~Rx,~Gx,~Bx,~Zx left slope, and x axis, respectively. -From the~e 18 input parameters, the triangle interpolator generates the fo].lowing 11 output parameters for triangle to be rendered, 7 of which are passed for each horizontal line primitive to be drawn:
Y - vertical address for the horizontal line XL - starting x value of the left end of the line XR - starting X value of the right ~ ~
end of the line - ;
R - initial red for line G - initial green for line B - initial blue for line ~R - delta red ~G - delta green ~B - delta blue ~
Z - initial z -~z - delta z Fig. 3A shows the relati~nship between the parameters in the triangle data structure input :
parameters and the rendered triangle primitive (a capital delta has been substituted for a ~). For examplej the top left-hand pixel of the triangle primitive (at XL and Ymin) has the initial color and depth values o Ro~ G
Bo and ZO. The left-hand pixel on the horizontal line below Ymin has the X value of XL + ~XL and the color and depth values of Ro + ~RS, Go + ~Gs~ O S O
Z S , Triangles are rendered as a ~eries of color interpolated 3-D lines. The triangle interpolator computes 11 output parameters and passes them to the line draw and parameter calculator. When the new triangle is to be rendered, the parameters for the first line are sent to the line draw and parameter calculator immediately because no interpolation is required. In ... . . . . .....

~T9-90-022 10 2 ~ ~ 3 9 4 7 subsequent triangle interpolation loops, seven new address and color parameters are generated in each pass.
The following overall sequence is performed by the triangle interpolator. First, the triangle interpol~tor receives the 18 parameters of the triangle primitive from the input interface. Secondly, the output parameters for a first line primitive are generated to fill one horizontal span of the triangle. Yl, Y2, XL, XR (with Yl and Y2 e~ual to Y) are passed to the bitblt and line draw parameter calculator 320 and R, G, B, ~R, ~G, ~B, Z, ~Z
are passed to the data path 370. Thirdly, new parameters XL, XR, Y, R, G, B, Z are calculated and stored for the next line primitive to be calculated. Steps 2 and 3 are then repeated until Ymax is detected, thereby indicating the last line primitive to be generated.
Fig. 3B is a diagram of a preferred embodiment of the triangle interpolation subsystem that is used to calculate the output parameters from the input parameters. Triangle interpolator 310 includes à buffer (register file) management controller 400 (TBF_FSM), a triangle interpolation and parameter handoff controller 401 (TRI_FSM~, an address mapper 402~ a comparator 403, thirt~ six 32-bit words of register file storage 404, a carry look-ahead adder 405 for interpolation and its accumulator register 406, and da-ta busses 407 to line generation subsystems (addressing, color and Z
interpolation). The address mapper is a lookup table which creates the physical register file addresses by decoding the inputs from buffer management controller 400, handoff controller 401 and input interface 300,. The comparator 403 detects an end-of-triangle condition.
Carry look-ahead adder 405 is used for interpolation.
Register file 404 has enough storage capacity to hold parameters for two triangle primitives. This allows triangle operations to be doubled buffered. That is, data for a new triangle is loaded into an idle portion of the buffer while the interpolation controller 401 utllizes the other half of the buffer. Buffer management controller 400 insures that the two buffers are kept separate. The register file has two write and three read , ~T9-90-022 11 2 ~ 5 3 9 ~ 7 data ports. One write port (W1) of the register file as assigned for loading new data. The other write port (W0) is used by the interpolation controller two store computation results. The new data can be loaded into the idle buffer and parallel with triangle interpolation tasks until the idle half of the idle bu~fer is full.
The -three read data ports are used for parameter passing and computation.
The TRI_FSM algorithm used in the interpolation controller 401 is tuned to further increases throughput.
The initial cycles of line generation need only the address information created b~ the triangle pass. The color parameters are not needed until line drawing actually begins. To take advantage o~ this, the triangle computation operations are sequenced such tha-t the line addressing parameters are generated first then the color and dep-th parameters next. At the beginning of the interpolation pass, the line generator is checked for availability. If the check is positive, a parameter passing se~uence different from the one described abové
is used. The address parameters are passed off one at a time directly from the accumulator and line sequencing is started while generation of color and depth parameters continues. The color and depth parameters are then passed at a later time. The parameter calculator 320 then processes the line addresses while the triangle interpolator continues processing color and depth information. The triangle controller can immediately begin the next pass or even a new triangle if the current triangle is compiete and the idle buffer has been filled.
Fig. 3C is a timing diagram that shows the parallelism achieved by the triangle interpolator. For purposes of clarity, a state diagram of TRI_FSM is included. The horizontal axis represents increasing time. The vertical axis indicates the tasks being performed in each cycle. For example, from time TO to time T1 the three line addressing parameters (Y0, XL0, XR0) are being transferred from the register file to the line draw parameter calculator while -three new register file addresses are being clocked into latches in , .:

2~53~7 -:'T9-90-022 12 preparation for a jump to time T2. At time T4 to time T5 the three delta color values are being transferred while the read address needed for incrementing Y is being prepared. At time T8 to time T9, XR is being stored, Zi is being computed and the addresses for computing Ri are being prepared.

Bitblt and Line Draw Parameter Calculator The bitblt and line draw parameter calculator 320 preprocesses addressing information for the bitblt and line draw sequencer 340. The sequencer 360 computes individual pixel addresses, controls color interpolation pacing and communicates with the memory hypervisor. ~y partitioning memory addressing into two tasks (an address preprocessing task and a pixel addressing task, each task to be performed by separate subsystems), a first line or bitblt need only be partially processed prior to starting processing on a second line or bitblt. This also applies to both Bresenham (see pages 74 to 81 of the second edition of FUND~MENTALS OF INTERACTIVE COMPUTER GRAPHICS
by J. D. Foley and A. Van Dam) and DDA (see pages 73 to 74 of the second edition of FUNDAMENTALS OF INTERACTIVE
COMPUTER GRAPHICS by J. D. Foley and A. Van Dam) line calculations. This is particularly useful when rendering ~
triangle primitives. ~`
The 4 line input address parameters are processed to produce line direction and quadrant information, as well as the Bresenham algorithm parameters. The bitblt inputs must be processed to determine which direction i.n X and Y
that pixel reading and writing must proceed such that pixels are not written over before being copied. The in~ut and output parameters for line primitives are:
Line Inputs:
Yl - y value at a first endpoint Y2 - y value at a second endpoint Xl - x value at a first endpoint X2 - x value at a second endpoint Line Outputs:
Ystart - Yl Yend - Y2 'T9-90-022 13 2053947 Xstart_~uot - Xl divided by five Xend_quot - X2 divided by five Xstart_rem - Remainder from division of Xl/5 Xend_rem - Remainder from division of X2/5 :-D_term, Il, I2 - Bresenham Algorithm Parameters Slope_GT_l - Slope of line is > 1 Slope_pos - Slope is positive ;.
Xl_GT_X2 - Xl > X~
The input and output parameters for bitblts are:
BitBlts Inputs:
Xsource - Lowest X address of the block to be copied Ysource - Lowest Y address of the block to be copied ~ :~
Xdestination - X address of upper left corner of block destination Ydestination - Y address of upper left corner of block destination .
Width - Number of pixels wide ~ . ;
Height - Number of pixels high :
BitBlt Outputs:
Xsrc_start_d5 - Starting X address for the :~
source block Xsrc_end_d5 - Ending X address for the ~ ~
source block ~ :`
Ysrc_start - Starting Y address or the source block Ysrc_end - Ending Y address for the source block ~
Xdest_start_d5 - Starting X address for the ~ -destination block ~- ;
Xdest_end_d5 - Ending X address for the destination block ~:
Ydest_start - Staring Y address for the destination block Ydest_end - Ending Y address for the destination block ~T9-90-022 14 2 ~ 5 3 9 4 7 Xmasks (4) - 5-bi-t Masks indicating which pixels in a 5-pixel group are to be manipulated at a block boundary.
The parameter calculator enables a mix of graphics primitives in any sequence. For example, a bitblt can precede a DDA lina, which can then precede a sequence of polyline segments, a series of triangle spans, etc. An idle state of the parameter calculator allows initialization to occur, thereby not wasting any cycles.
In addition, an final state of the parameter calculator, where output parameters are passed to the sequencer, performs the final parameter calculations. As a result, the last output values are passed directly to the sequencer rather than to registers within the parameter calculator. This subsystem can also per~orm up to five arithmetic operations simultaneously during some -processing steps [including ta+b), (a-b), (b-a), (a-b~*2, a-b*2, a*2, a<b, a>b, a=b, a/5] to maintain high throughput.
Fig. 4A is a block diagram of the bitblt and line draw parameter calculator dataflow. The input parameters are received from input interface 300 and triangle interpo-lator 310. The output parameters are then calculated from the input parameters by multiplexers 510, inverters 520, carry look-ahead adders 530, divider 540 and registers 550.
Fig. 4B is a state flow diagram for the parameter ~ ;
calculator 320, with detail and emphasis on a Bresenham line sequence. This controller switches the multiplexers and registers depic-ted in Fig. 4A to complete the parameter calculation sequence. For example, step ~-slp_gtl, the variable dx is compared to dy. I f dy is greater than dx, then the slope of the line to be drawn creates an angle greater than 45 degrees, so the slope_gt_one flag is set. In step d_term, the slope_gt_one flay is tested to determine how the dterm variable is calculated, as shown in the comments accompanying the figure. ~
:`

~T9-90-022 15 2 0 ~ 3 9 ~ 7 Bitblt and Line Vraw Sequencer The bitblt and line draw se~uencer calculates frame buffer addresses, controls, color interpolation pacing and communicates with the memory hypervisor. The se~uencer works with data path 370 to pass address information from the sequencer and corresponding data from the data path in a coordinated manner.
The input address parameters are processed to produce pixel addresses and write enables, using either the Bresenham or DDA algorithm for lines. The bitblt inputs determine which direction in X and Y that pixel reading and writing must proceed such that pixels are not .
written over before being copied. The input and output parameters for the sequencer are: ~
~ , Line Inputs: :~
Ystart - Yl Yend - Y2 Xstart_quot - Xl divided by five Xend_quot - X2 divided by five ! Xstart_rem - Remainder from division ::
Xend_rem - Remainder from division :~
D, Il, I2 - Bresenham Algorithm .
Parameters Slope_GT_l Slope_pos - j ~
Xl_GT_X2 - .
Bitblt Inputs:
Xsrc_sta~t_d5 - Starting X address for the :~
source block -Xsrc_end_d5 - Ending X address for the ~:
source block Ysrc_start - Starting Y address for the :~
source block Ysrc_end - Ending Y address for the source block Xdest_start_d5 - Starting X address for the destination block Xdest~end_d5 - Ending X address for the destination block :~

. ~ ,, : ...
, ~T9-90-022 16 2 0 5 3 9 ~ 7 Ydest_start - Staring Y address for the destinatîon block Ydest_end - Ending Y address for the destination block Xmasks (4) - 5-bit Masks indicating which pixels in a 5-pixel group are to be manipulated at a block boundary.
Outputs:
PixXaddr - 8-bit X frame buffer address PixYaddr - 10-bit Y frame buffer address WeMask - 5-bit pixel write enable mask MemReadReq - Frame buffer read request MemWriteReq - Frame buffer write request DestNotSource - Flag indicating if operation is destination or ~ source.
Fig. 5 is a block diagram of the bitblt and line draw sequencer dataflow. Bitblt and line draw parameter calculator 320 is shown providing data to the bitblt and line draw sequencer 360. Sequencer 360 includes addressing control 610, arithmetic and logic unit~ 620, 630 and 640 (also known as arithmetic units), memory request control 650 and memory interface 660. The arithmetic units perform the necessary calculations to draw the primitives from the data provided by the parameter calculator. The arithmetic units 620 and 630 handle the Y variables and arithmetic unit 640 handles the X variables. Addressing control 610 controls the operation of the arithmetic units and the result~ of their operations being passed to the memory interface.
Memory request conkrol 650 controls the operations of the memory interface. The memory interface stores and formats data for being passed to the memory hypervisor.
In addition, the operation of the memory request control and the memory interface allows the addressiny control .
'; ,,, : ,,, ~T9-90-022 17 2~3947 and the arithmetic units to function at high spee~ in parallel.

~put Interface Fig. 6A is a more detailed block diagram of the input interface 300 shown in Fig. 2. An address decoder 700 includes a register 702 and a decodex 704. The decoder determines what type of request is pending and classifies the requests into different types such as overall re~uest, triangle request, etc. There are also miscellaneous requests that are never protected. That is, an interrupt register can always be read or written. The request, read/write and strobe lines from the front end processor 210 and the busy signals from the rasterization processor functional blocks are all inputs to a register protection checker 710. The register protection checker includes an interface 712 and an interface controller state machine 714. The checker compares the type of re~uest with the current status of the busy lines to determine if the register operation can procaed. When the operation can proceed the ready line is asserted to the front end processor and (if the register operation is a write) the write strobe line is asserted to the correct functional block.
Fig. 6B is a state diagram of the input interface in operation. The command interpreter and supervisor 340, trian~le interpolator 310, bitblt and line draw parameter calculator 320, and data path 370 each provide a busy signal to the input interface indicating when their internal registers are free to be read or written. When an internal register is written (or read) by the front end processor, the input interface determines if the operation may proceed by testing a busy signal from the associated functional block. If the register operation may proceed, then the input interface returns a ready signal to the front end processor. If the register operation is a read, then the data is sent to the front end processor. If the register operation is a write, then the internal register is strobed and the new data is loaded into it. If the register operation cannot . . , ~9-90-022 18 20539~7 proceed, then the input interface withholds ready until the functional block permits the operation. The status of the busy signal of each block can be more than just a simple busy indicator. If the block has an input FIF0, then the busy can correspond to a FIF0 full signal.
The register protection scheme improves overall performance by eliminating the testing of a status register. Whenever the next register load can proceed, the time spent testing the status register and determining that the register can be loaded is ~asted.
The performance improvement is most dramatic when only a few parameters need to be loaded for an operation and the operation is quick (such as a short line). In these cases, the time taken to read the status register indicating not busy is large relative to the time to load ~he parameters.
The register protection scheme also facilitates overlapping operations. For exa~ple, while the bitblt and line dra~ parameter calculator 320 is completing a line, parameter~ can be loaded into the triangle interpolator. This incr~ases the performance when there is a mixture of operations to be performed. When starting an operation, the command interpreter and supervisor 340 has a separate busy signal, thereby preventing update of the start operation register until the previous operation is complete.
While some registers are specific to one type of operation, others are common to more than one. The input interface allows each internal register to be protected by one or more busy signals. This allows, for example, the XY address of a line to be protected solely by the line drawer busy signal while the destination buffer selection (frame buffer, Z buffer or attribute buffer) requires all the busy signals to be inactive to prevent changing the buffer selection in the middle of an operation. Providing this differentiation allows a simple DMA controller to load any register in the rasterization processor in any order. The input int~rface prevents a register from being loaded at an improper time. The DMA controller can be set to copy from a ~T9-90-022 19 ~0 ~ 3 9 ~ 7 display list in memory to the rasterization processor.
As the rasterization processor completes operations, the next set of parameters can be loaded. Different operations can be mixed, as well as changes to overall control registers (buffer selection for example).
By separating address and data busy signals, such as the busy from the bitblt and line draw parameter calculator 320 and the busy from the data path 370, fast characters (bitblt) can be processed with a simple DMA
interface. In this case, the protection mechanism allows the address information ~destination X, Y) of each successive character to be entered while the previous character bitblt is completed. Then the protection mechanism will feed the data path at the rate it can handle the new character image.

Memory Hyp rvisor There are several ways to implement Z and attribute compare in hardware. One is to have a single memory controller for the frame, attribute and Z buffers. The rasterizer front end generates address, color and Z depth information, decides which bufer currently needs to be accessed, and sends a request to the memory controller to access the appropriate buffer. When the memory controller has finished, the rasterizer sends the next request to be processed. While æ and/or attribute compare is active, several memory request~ mu~t be sent by the rasterizer front end to write a single pixel to the destination.
The performance of this implementation is limited primarily for three reasons. The first is that all buffer accesses must be done sequentially because there is only one memory controller. The second reason is that the process of reading the Z and/or attribute buffers, sending the data to the rasterizer front end, doing compares, dec.iding which buffer to access next, and sendi.ng the appropriate request to the memory controller re~uires many pipeline stages. This series or loop of operations must be performed for every pixel. The time required to go through this loop is almost always too ~T9-90-022 20 ~ 3 9 ~ 7 long to operate at maximum memory band-width. The third reason is that the rasterizer front end cannot start rasterizing (i.e., generating address, color and Z depth) the next pixel until it finishes doing the Z and/or attribute compares and writes the current pixel to the destination buffer. Conversely, the Z and/or attribute compares cannot be performed unless that pixel has been rasterized. Since the time required to rasterize a pixel and to access a buffer ~aries, there will be many instances where the memory banks will be waiting for a pixel to be rasterized or the rasterizer will be waiting for the memory accesses to complete. These wait states reduce overall performance.
Fig. 7A is a more detailed block diagram of the memory hypervisor shown in Fig. 2. Parallel independent memory controllers 800, 810 and 820 are used for Z, attribute buffers, respectively. This makes it possible to execute multiple memory re~uests simultaneously such as reading the attribute buffer while writing the frame buffer. Even if two memory requests cannot be executed simultaneously because the buffers share data busses, it is possible to overlap the memory controllers such that the shared busses are run at maximum speed.
A hypervisor 830 is used to initiate and control the compare operations and memory controllers. It is close (i.e., few pipeline stages) to the memory controllers, -thereby allowing the compare results to be utilized quickly. This allows us to more efficiently utilize the data busses.
Rasterizer front end requests are placed into a pipeline or buffer 840 in front of the hypervisor. This allows smoothing of the variations in performance between the rasterizer front end and the memory banks, ther by allowing each to operate at maximum speed more often.
This reduces the number of instances where the memory banks are waiting for a pixel to be rasterized or the rasterizer front end is waiting for the memory accesses to complete. It also allows the hypervisor to start execution of the next memory operation while executing the current operation.

~T9-90-022 21 20~9~7 Read and write requests are sent by the rasterizer front end to the hypervisor. When the hypervisor is ready to accept a request, it will send a grant to the rasterizer front end and latch all necessary data associated with the request. Upon receiving a yrant, the rasterizer front end considers the request complete and begins generating the next re~uest, even though the hypervisor has not completed the first request.
In addition to read and write operations, refresh operations must be done on both VRAMs and DRAMs to ensure memory data integrity. Also, serial register load operations must be done on the VRAMs in order to display the memory contents on a graphics display. When either or both of these operations need to be performed, an interrupt is sent to the hypervisor. The hypervisor will complete the front end read or write request, cease operation until the required refresh/serial load operations are completed, and then resume processing rasteri~er requests.
The pipelining, synchronization and sequencing of the memory requests are better understood with a few examples. Fig. 7B shows how the apparatus of Fig. 2 would process a series of writes to the frame buffer with Z and attribute compare being performed. First the rasteriæer front end (Fig. 2) puts a write request into the pipeline. The hypervisor immediately determines which compare options must be performed (in this example, both attribute and Z). On the next cycle, a read request is sent to the attribute memory controller. Since this controller is inactive, it immediately sends a grant to the hypervisor and latches the address. The hype~visor considers the operation done upon receiving the grant and, on the next cycle, sends a read request to the Z
memory controller. In this example, the Z and frame buffer memory share a common data bus, therefore the Z
memory controller must wait until the frame buffer memory controller is done with the data bus. This wait period is determined by passing various busy signals between all the memory controllers. After the appropriate wait period, the Z memory controller sends a grant to the ~T9-90-022 22 2 ~ 5 3 9 ~ 7 hypervisor. Next, a write re~lest is sent to the Z memory controller. If the Z and/or attribute compare is false, then the Z memory controller will abort the write operation after sending the grant. Finally, a write re~uest is sent to the frame buffer controller. Again, if the compare is false, the frame buffer controller will abort the write operation after sending the grant.
Fig. 7C shows how the rasterizer would process a series of writes to the frame buffer with only an attribute compare being performed. First, the rasterizer front (Fig. 2) end puts a write request into the pipeline. The hypervisor immediately determines which compare options must be performed (in this example, only attribute compare). On the next cycle, a read request is sent to the attribute memory controller. Since this controller is inactive, it immediately sends a grant to the hypervisor and latches the address. Upon receiving this grant, the hypervisor simultaneously sends a write request to the frame buffer memory controller and a read request to the attribute frame buffer (this attribute read request is for re~uest N+l in the pipeline, which is obtained one pipeline staqe before the regular processor request). If the attribute compare is false, the frame buffer controller will abort the write operation after sending the grant.
. . ..
Timing Di~Lrams `' The operation of the pipeline and interlocks are better understood with a few examples. Fig. 8A shows how the rasterizer 220 of Fig. 2 would process a series of lines followed by a series of bitblts. The boxes denote when each subsystem is busy. The shading is to aid in differentiating between adjacent commands. For a line the se~uence is as follows. The end points of the line are loaded through the input interface 300, the final value is a go signal to the command interpreter 340, the command interpreter starts the parameter calculator 320 if it i6 not either busy or waiting for its last set of calculated parameters to be u~ed, the parameter calculator creates necessary values for the draw : ~

~T9-90-022 23 2 ~ ~ 3 9 4 7 sequencer 360, and the draw sequencer makes memory requests to the memory hypervisor 380. As soon as one subsystem completes the first line, it is free to work on the next line. Thus, the parameter calculator works on line 2 while the drawing sequencer is working on line 1.
Once the first line is started, the memory hypervisor is kept busy. If line 3 is followed by a bitblt, the command interpreter starts the parameter calculator on the bitblt even before the drawing sequencer is finished with a line. Therefore, two different styles of commands are processed in an overlapped fashion and the m~mory hypervisor maintains maximum utilization.
Fig. 8B shows how the apparatus of Fig. 2 would process a series of triangles. The shading is to aid in differentiating between operations related to adjacent triangles. The sequence is as follows. The parameters of the triangle are loaded through the input interface, the final value is the go signal to the command interpreter, the command interpreter starts tria~gle interpolator 220 if it is not busy, the triàngle interpolator feeds a series of lines (or spans) to the parameter calculator if it is free, the parameter calculator creates the necessary values for the draw sequencer and the draw sequencer makes memory requests to the memory hypervisor. There are two triangle parameter buffers, so as soon as one triangle is loaded, a second can also be loaded. The third triangle cannot be loaded until the first is completely interpolated. Once the first span of a triangle is started, the memory hypervisor is kept busy. The triangle interpolator has internal storage for two sets of parameters so that there is not an unused stretch of time after one triangle is done while the input interface is busy receiving new parameters for the next triangle.
Fig. ~C shows how the apparatus of Fig. 2 would process a series of external bitblts which re~uire format conversion of the input data (such as a character expansion). The shading is to aid in differentiating between operations related to adjacent bitblts. The sequence is as follows. The parameters of the bitblt are ~T9-90-022 24 2~3947 loaded through the input interface, the final value is the go signal to the command interpreterJ the command interpreter starts the parameter calculator if it is free, the parameter calculator creates the necessary values for the draw sequencer and the format converter 330, the bitblt data can now begin to come via the input interface which passes it to the format converter, and the draw sequencer makes memory requests to the memory hypervisor once the data path 370 has the data from the format converter. The format converter has an internal FIFO so that it can get ahead of the draw sequencer, thereby allowing the input inter~ace to finish early.
Then the parameters for bitblt 2 can be loaded restarting the whole sequence. If the format converter FIFO is large enough and the input interface loads faster than the memory hypervisor can write the pixels, then the memory can be kept utilized. Certainly for characters this is true since for each bit of input data a pixel of output data is generated which is typically 3 bytes (24-bit pixels).
Overlapping operations are supported by having interlocks among the various state machines which allow subsequent operations to begin before the current operation is complete whenever possible. The input interface allows new values to be loaded in registers that are either currently not used or no longer raquired to be stable. The command interpreter determines which subsystems are involved in the previous command and allows a new command to proceed if the first active block for the new command is not busy processing the previous command. For example, as shown in Fig. 8D, if the new command is a triangle and the previous command was a line (a bitblt would similar), then the command interpreter would pass the triangle interpolator the start triangle signal. The triangle interpolator would then proceed to create the first horizontal span of the triangle, but parameter calculation for that span would be held off until the bitblt and line draw parameter calculator was free. If the line is long enough (or the bitblt large enough), then the input interface loads the triangle ' ~

~T9-90-022 25 2 0 ~ 3 9 4 7 before the memory hypervisor completes the previous operation. The first line of the triangle can also be calculated while the previous line is being drawn, keeping the memory utilized. However, when a bitblt follows a triangle, the input interface prevents the bitblt parameters from being loaded until after the last line of the triangle is completed. This would leave a gap in the memory utilization depending on the length of the last line. In Fig. 8D, a small gap is shown, but most lines are long enough so that no gap would exist.
Although the present invention has been fully described above with reference to specific embodiments, other alternative embodiments will be apparent to those of ordinary skill in the art. For example, the Bresenham algorithm for line draws can be broken into more than two sequential tasks that can be performed by more than two subsystems. Therefore, the above description should not be taken as limiting the scope of the present invention which is defined by the appended claims.

~, ":. :

Claims (3)

?T9-90-022 The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A graphics processor comprising:
a) means for providing triangle primitives representing a graphics image;
b) means, coupled to said providing means, for interpolating a triangle primitive and serially computing a plurality of line primitives therefrom;
c) means, coupled to said interpolating means, for receiving a line primitive from the interpolating means, and for providing pixels representing said line primitive while said interpolating means is computing a next line primitive, and d) means for providing said pixels for display.

2. An system for rendering graphics comprising:
a) a host computer;
b) a graphics processor coupled to the computer, the graphics processor including:
i) means for providing triangle primitives representing a graphics image;
ii) means, coupled to said providing means, for interpolating a triangle primitive and serially computing a plurality of line primitives therefrom;
iii) means, coupled to said interpolating means, for receiving a line primitive from the interpolating means, and for providing pixels representing said line primitive while said interpolating means is computing a next line primitive; and iv) means for providing said pixels for display.

3. A method of rendering a triangle comprising the steps of:
a) providing a triangle primitive representing a graphics image;
b) interpolating the provided triangle primitive and serially computing a plurality of line primitives therefrom;

c) providing pixels representing said line primitives while said a next line primitive is being computed; and d) providing said pixels for display.